There are many applications where it is necessary to cancel or reject an undesired signal component of an input signal. One such application relates to the processing of a radar return signal. In this application, the radar return signal will be accompanied by an unwanted signal, such as a backscatter signal component, which typically has a significantly larger amplitude than the radar return signal. The frequency excursion of the backscatter signal is usually predictable. Accordingly, a notch filter which is designed for this particular backscatter frequency band provides a convenient way of rejecting the backscatter signal component during the processing of the radar return signal.
One type of notch filter which has been used to reject an undesired signal component has been referred to as a quadrature notch filter. A quadrature notch filter works by adding two amplitude and phase controlled reference signals to the incoming or radar return signal in such a way as to cancel the unwanted signal component. However, this type of notch filter has generally been limited to relatively narrow band width applications (e.g., below 1 MHz). This limitation rises because 4-quadrant multipliers must be used in the filter, and the response of these devices is non-linear during swings between positive and negative phase transistions. This limitation has made it difficult to use quadrature notch filters in relatively wide band width applications, such as the rejection of jamming signals in radar systems and infrared or laser sensors. In such applications, the phase of the backscatter signal can shift quite rapidly even though the frequency band width of the backscatter signal is quite limited. Thus, it would be desirable to provide an enhanced quadrature notch filter circuit which can adapt to relatively high frequency phase shift rates of an undesired or backscatter signal component.